Metals or alloys react with hydrogen exothermically to produce metal hydrides, and the metal hydrides reversibly release hydrogen gas endothermically. LaNi5Hx, MmNi5Hx, MmCo5Hx, FeTiHx, VNbHx and Mg2CuH are common examples of metal hydrides which have the ability to occlude a significant amount of hydrogen and release a large amount of the heat of reaction. Various metal hydride devices are known, such as heat pumps and air conditioning devices, which utilize these properties of the metal hydrides to provide heating and/or refrigeration. In these metal hydride devices, hydrogen is used as a refrigerant and metal hydrides are used as absorbents.
A conventional metal hydride heat pump comprises a first receptacle filled with a first metal hydride, a second receptacle filled with a second metal hydride, the first and the second metal hydrides having different equilibrium dissociation characteristics; a hydrogen flow pipe connecting these receptacles; and heat exchangers in the respective receptacles. Typically, a heating output and a cooling output, based on the heat generation and absorption of the metal hydrides within the receptacle, is obtained by means of a medium flowing within the heat exchangers.
The metal hydride heat pump operates in a cyclic nature. A pair of two different types of metal hydrides are used, viz., regenerating alloy A and refrigerating alloy B, as sorbents, and hydrogen as a refrigerant. In the first cycle of operation of paired reactors of alloys A & B, alloy A discharges hydrogen using a first medium of high temperature heat source. The discharged hydrogen is absorbed by the alloy B and in the process heat is rejected to a second medium, typically ambient air. In the second cycle alloy B desorbs hydrogen using a third stream of low temperature heat source. The discharged hydrogen is absorbed by alloy A and in the process heat is rejected to the fourth stream, typically ambient air. Thus, the operation of the metal hydride heat pump requires each alloy to go through a temperature swing for charging and discharging.
Conventionally, dampers are used for the changeover. The dampers, ducting and the casing of the heat pump form a part of the thermal cycling, which results in increased thermal inertia. U.S. Pat. No. 6,722,154 suggests a metal hydride based air cooling method and apparatus. The apparatus comprises an intricate network of air conduits and dampers. This results in an increased thermal inertia. Higher thermal inertia is highly undesirable for the system and results in reduced performance. The arrangement of reactor casings connected with multiple dampers by interconnecting ducting requires multiple bends and higher flow length for the air streams used as the heat transfer medium. This results in higher pressure drop in the system, requiring higher power for the air fans and blowers. Furthermore, the air distribution is not uniform resulting in reduced performance. The multiple dampers are connected to the reactor casing by interconnecting ducting which makes the system bulky and heavy. Also, the interconnecting ducting results in increased height of the system, which is undesirable for applications such as mobile air conditioning in vehicles, due to the increased drag force on the vehicle.
There is therefore need for an air changeover system for metal hydride heat pumps that overcomes the above-noted drawbacks of conventional air changeover systems in metal hydride heat pumps.